1. Field of the Invention
The present invention relates to an encoder employing a method of measuring a displacement of an optical scale in which a relief type diffraction grating is formed on a light-transmission substrate by utilizing diffracted light produced by the diffraction grating.
2. Related Background Art
A device disclosed in, e.g., Japanese Utility Model Publication No. 61-39289 is known as a conventional encoder such as a linear encoder, a rotary encoder, or the like. In this device, as shown in FIG. 8, grooves are cyclically formed on a transparent glass substrate to form a relief type diffraction grating. A metal reflection film of Au or Al is deposited on the cyclic groove surfaces to constitute an optical scale. The optical scale is illuminated from a position above the relief type diffraction grating, and high-order diffracted light components produced by the relief type diffraction grating are caused to interfere with each other, thereby forming interference fringes. A change in intensity of the interference fringes is detected and is photoelectrically converted to obtain a sinusoidal signal. A displacement of the optical scale is measured based on the sinusoidal-wave signal.
The relief type diffraction grating described above is very effective since the height of the groove can be appropriately selected to weaken the intensity of 0th-order reflected/diffracted light (regularly reflected light) and to enhance the intensity of high-order reflected/diffracted light used in measurement.
As shown in FIG. 8, however, when a reflection film 3 is deposited on the groove surfaces of the relief type diffraction grating, the deposited film has a nonuniform thickness in a strict sense, and the grooves have nonuniform shapes and depths, accordingly. As a result, the intensity of diffracted light is varied, and it is difficult to perform high-precision measurement.